Awake and hungry: artificial light at night disrupts behaviour and reproductive ecology in a wild migratory bird

Artificial light at night (ALAN) disrupts biological rhythms across taxa, yet behavioural mechanisms linking exposure to population consequences remain poorly understood. Long-distance migrants encounter ALAN across their annual cycle but remain understudied despite experiencing stronger performance impacts than sedentary species. We experimentally introduced ALAN into nestboxes of a migratory passerine, the collared flycatcher (Ficedula albicollis), breeding in Gotland, Sweden. Nestlings were exposed to ALAN from two days post-hatching until fledging, and we video-recorded parental and nestling activity over 24 hours on day eight post-hatching. Across a high-resolution behavioural dataset (32,100 nestling and 3,709 parental events), ALAN-exposed nestlings begged more frequently and for longer at night compared to dark controls, revealing disrupted circadian activity. These effects cascaded to parents: both females and males began feeding earlier, and ceased later, while reducing hourly feeding rates relative to controls. Consequently, ALAN nestlings fledged at older ages, consistent with delayed development, though reproductive success (number of fledged offspring) was unaffected. Our study provides clear empirical support for socially mediated behavioural mechanisms whereby both parents respond to ALAN, disrupting family-level coordination, developmental trajectories, and parental investment. This temporal destabilisation of social synchrony uncovers a novel ecological pathway through which anthropogenic light alters behavioural timekeeping and life-history trajectories in the wild.